A Cautionary Tale: small earthquakes that might have changed our
understanding of Tibetan geodynamics — but were mis-located
Abstract
Earthquake moment tensors and centroid locations in the catalogue of the
Global CMT (gCMT) project, formerly the Harvard CMT project, have become
an essential and extraordinarily valuable resource for studying active
global tectonics, used by many solid-Earth researchers. The catalogue’s
quality, long duration (1976–present), ease of access and global
coverage of earthquakes larger than about Mw~5.5 has
transformed our ability to study regional patterns of earthquake
locations and focal mechanisms. It also allows researchers to easily
identify earthquakes with anomalous mechanisms and depths that stand out
from the global or regional patterns, some of which require us to look
more closely at accepted interpretations of geodynamics, tectonics or
rheology. But, as in all catalogues that are, to some extent and
necessarily, produced in a semi-routine fashion, the catalogue may
contain anomalies that are in fact errors. Thus, before re-assessing
geodynamic, tectonic or rheological understanding on the basis of
anomalous earthquake locations or mechanisms in the gCMT catalogue, it
is first prudent to check those anomalies are real. The purpose of this
paper is to illustrate that necessity in the eastern Himalayas and SE
Tibet, where two earthquakes that would otherwise require a radical
revision of current geodynamic understanding are shown, in fact, to have
gCMT depths (and, in one case, also focal mechanism) that are incorrect
— in spite of the overwhelming majority of gCMT solutions in that
region being unremarkable and likely to be approximately correct.